Try this
a=0``-0.5
ListPlot[Table[{x, a + x}, {x, -10, 10}], Joined -> {True}]
This is probably not a surprise to some people.
If you think the value for "a" could never happen, try this:
a=N[1,20]
Do[a=2*a-a,{43}]
Compare to Plot[x+a,{x,-1,10}]
which shows that Plot evaluates its first argument in an manner
inconsistent with Table's method.
Cheers.
RJF